[gtk/macos-ci: 2/3] macos: fix typedef on older macOS

[Christian Hergert <chergert src gnome org>]

commit f4f104c9f9641418b66c59071997e0faf4ba4754
Author: Christian Hergert <chergert redhat com>
Date:   Tue Jan 5 13:53:22 2021 -0800

    macos: fix typedef on older macOS
    
    On older systems, we don't have a typedef for gravity. They are a NSString
    pointer and we can provide that manually just fine.

 gdk/macos/GdkMacosWindow.c | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/gdk/macos/GdkMacosWindow.c b/gdk/macos/GdkMacosWindow.c
index f4fcd01ad5..3afcdeb826 100644
--- a/gdk/macos/GdkMacosWindow.c
+++ b/gdk/macos/GdkMacosWindow.c
@@ -38,6 +38,10 @@
 #include "gdkmonitorprivate.h"
 #include "gdksurfaceprivate.h"
 
+#ifndef AVAILABLE_MAC_OS_X_VERSION_10_15_AND_LATER
+typedef NSString *CALayerContentsGravity;
+#endif
+
 @implementation GdkMacosWindow
 
 -(BOOL)windowShouldClose:(id)sender